Kinetic energy release and position of the transition state during the intramolecular substitution of ionized benzalacetones

Proximity effects in the electron ionisation mass spectra of substituted cinnamamides

The electron ionisation mass spectra of an extensive set of 53 ionised monosubstituted and disubstituted cinnamamides [XC₆H₄CH=CHCONH₂, X = H, F, Cl, Br, I, CH₃, CH₃O, CF₃, NO₂, CH₃CH₂, (CH₃)₂CH and (CH₃)₃C; and XYC₆H₃CH=CHCONH₂, X = Y = Cl; and X, Y = F, Cl or Br] are reported and discussed. Particular attention is paid to the significance of loss of the substituent, X, from the 2-position, via a rearrangement that is sometimes known as a proximity effect, which has been reported for a range of radical-cations, but is shown in this work to be especially important for ionised cinnamamides. When X is in the 2-position of the aromatic ring, [M - X]⁺ is formed to a far greater extent than [M - H]⁺; in contrast, when X is in the 3-position or 4-position, [M - H]⁺ is generally much more important than [M - X]⁺. Parallel trends are found in the spectra of XYC₆H₃CH=CHCONH₂: the signal for [M - X]⁺ dominates that for [M - Y]⁺ when X is in the 2-position and Y in the 4-position or 5-position, irrespective of the nature of X and Y. Further insight is obtained by studying the competition between expulsion of X^· and alternative fragmentations that may be described as simple cleavages. Loss of ^·NH₂ results in the formation of a substituted cinnamoyl cation, [XC₆H₄CH=CHCO]⁺ or [XYC₆H₃CH=CHCO]⁺; this process competes far less effectively with the proximity effect when X is in the 2-position than when it is in the 3-position or 4-position. Additional information has been obtained by investigating the competition between formation of [M - H]⁺ by the proximity effect and loss of CH₃^· by cleavage of a 4-alkyl group to give a benzylic cation, [R¹R²CC₆H₄CH=CHCONH₂]⁺ (R¹, R² = H, CH₃).